Mechanism of N2 Reduction to NH3 by Aqueous Solvated Electrons

Recently a novel approach to the photocatalytic reduction of molecular nitrogen under ambient conditions was reported in which hydrated electrons generated from ultraviolet illumination of diamond served as the reducing agent Zhu, D.; Zhang, L.; Ruther, R. E.; Hamers, R. J. Photo-Illuminated Diamond as a Solid-State Source of Solvated Electrons in Water for Nitrogen Reduction. Nat. Mater. 2013, 12, 836-841. This surprising reduction of N2 by aqueous solvated electrons is absent from the vast existing radiolysis literature and thus has little mechanistic precedent. In this work, a combination of experimental and computational approaches is used to elucidate the detailed molecular-level mechanistic pathway from nitrogen to ammonia. A variety of approaches, including electronic structure calculations, molecular dynamics simulations, kinetic modeling, and pH-dependent experimental measures of NH3 and competing H2 production, implicate a hydrogen atom addition mechanism at early reduction steps and sequential protonation/direct reduction by a solvated electron at later steps, thus involving both direct and indirect reactions with solvated electrons. This work provides a framework for understanding the possible application of solvated electrons as energetic reducing agents for chemically inert species under mild conditions.